Old English clæne "free from dirt or filth; pure, chaste, innocent; open, in the open," of beasts, "ritually safe to eat," from West Germanic *klainoz "clear, pure" (cognates: Old Saxon kleni "dainty, delicate," Old Frisian klene "small," Old High German kleini "delicate, fine, small," German klein "small;" English preserves the original Germanic sense), from PIE root *gel- "bright, gleaming" (cognates: Greek glene "eyeball," Old Irish gel "bright").

"Largely replaced by clear, pure in the higher senses" [Weekley], but as a verb (mid-15c.) it has largely usurped what once belonged to cleanse. Meaning "whole, entire" is from c.1300 (clean sweep in the figurative sense is from 1821). Sense of "innocent" is from c.1300; that of "not lewd" is from 1867; that of "not carrying anything forbidden" is from 1938; that of "free of drug addiction" is from 1950s. To come clean "confess" is from 1919, American English.